Buildings have traditionally been designed to support their own weight plus that of expected inhabitants and furnishings. Buildings and other structures for supporting weight have long been expected to be very strong under vertical compression. Concrete is a favorite material for weight-bearing structures because it is inexpensive and has exceptional compressive strength.
In the mid-1900s, architects began to take lateral forces into account more than they had previously. Wind can exert strong lateral force on tall buildings and long bridges. Smaller structures were still designed without much regard for strong lateral forces, though, until concern for earthquake resistance began growing in the 1970s in the United States, partly due to the massive Anchorage earthquake in 1964.
Frame structures consist of a skeleton of elongate wood, metal, or concrete members that are connected together. These elongate members may be connected together by various means. In some cases brackets that join elongate members while resisting twist are used. More typically, framing members are connected with nails or screws that are easily bent and that can allow the framing members to pivot about the connection when under stress from an unusual direction.
Once the framework skeleton is complete, a sheathing of some material is applied over the framing to give a smooth surface and to increase the shear resistance of the wall. Such a sheathing material is typically plaster, wood paneling including plywood, or gypsum board, also known as drywall or sheetrock.
The stiffness of the sheathing material helps maintain the framework erect under lateral forces such as earthquake or high wind. Building codes take this effect into account and allow designers to include fewer diagonal braces or other shear reinforcements than would be required for unsheathed frame walls. Because various sheathing materials are known to have different shear strength values, there are different code requirements for constructing the frame, depending upon the planned sheathing material.
The shear strength values were formerly derived from small scale mechanical tests of the materials themselves. Testing of construction materials has become more realistic and sophisticated in the past few decades. As a result, some of the previously used strength values have been found to be inaccurate.
In particular, buildings that use gypsum board for interior walls, also called drywall construction, have been found to have much less resistance to lateral forces than their designers intended. For many years, designers used an erroneously high value for the shear resistance of walls faced with gypsum board. Later research, as well as analysis of buildings damaged by earthquake or wind, has shown that the true shear resistance contribution of gypsum board is only about 10% of what was previously accepted.
Many buildings worldwide need to be retrofitted so as to have the desired degree of shear force resistance. A conventional method for strengthening such buildings is to pull out the gypsum board and replace it with plywood that is attached to the building framework.
Replacing gypsum board with plywood is an effective method for increasing the resistance to lateral forces, but has disadvantages. The “demolition” step of removing the gypsum is extremely dusty, releasing particles into the atmosphere of the building and generating larger particles that drop to flat surfaces and into crevices. Between disposal of the bulk of the gypsum board and the cleanup of the building, a great deal of solid waste is created.
The dust may include gypsum, asbestos, and paper. Because dust in the air is harmful to people, animals, and many machines, the contents of the building have to be wrapped, packed, or removed so they are not contaminated. Residents or workers in the building being retrofitted may be required to absent the building for a day or longer.
Both the steps of demolition and of installing plywood are noisy for the entire duration of the work. Even if people and machines in the building can be isolated from the dust by temporary walls, such as of plastic sheeting, it is likely that the noise of the operation would prevent occupants from working or resting in the building during the retrofitting.
Simply replacing the drywall sheathing of interior walls with plywood may not be enough to increase the strength of the structure as much as desired. Additional wooden bracing within the walls or use of metal tie straps to connect various components of the structure together may be needed.
To withstand lateral forces such as seismic or wind forces, a structure's components must be strongly connected together. Yet, it has been found that extremely rigid structures do not fare as well in earthquakes or wind as structures with some flexibility. Replacing gypsum board with nailed-in plywood does not significantly improve the ductility of the structure. If additional internal bracing or metal connectors must be installed, the ductility of the structure may be actually reduced, leaving the structure still vulnerable to cracking or rupturing under strong lateral force. A violent failure of one component of a structure often causes a sudden chain reaction failure of other components, possibly trapping or crushing occupants. A ductile structure is more likely to fail in a gradual manner, allowing time for occupants to notice the impending failure and take steps to evacuate.
Seismic retrofitting by replacing gypsum board with plywood is expensive and is therefore typically being done on only the highest-risk structures. Costs of the method include loss of productivity and use of the building during the retrofitting, potential cost of temporarily relocating occupants, dust abatement and cleanup, cost of demolition labor and disposal fees for the gypsum, cost of protecting contents of the building, cost of additional bracing and reinforcing, and cost of the plywood itself and its installation. Lastly, after the walls are replaced, paint, trim, wallpaper and other ornamental finishes must be replaced.
The need for a less costly method of seismic retrofitting of drywall structures is great. Such a method should provide shear resistance that is at least equivalent to that of plywood. There is a need for such a retrofit method that does not generate large quantities of solid waste and that does not contaminate the structure with harmful dust and particles.
There is further a need for a retrofit method that can be performed while people work or live in the building, without undue noise or exposure to harmful materials. Such a retrofit method should preferably make it more likely that any failure of the structure, if it does occur, is gradual instead of sudden and catastrophic so that occupants may escape.